Electrical device protection of the hottest asynch

2022-08-14
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Electrical device protection of asynchronous motor

the protection of asynchronous motor is a complex problem. In actual use, appropriate protective devices and starting equipment should be selected according to the different capacity, type, control mode and distribution equipment of the motor

relationship between motor protection and control

motor protection is often related to its control mode to prevent rust, that is, there is control in protection and protection in control. If the motor is directly started, the starting current of 4-7 times the rated current will be generated towards the stroke. If it is controlled by a contactor or circuit breaker, the contact of the electrical appliance should be able to withstand the on and off assessment of the starting current. Even the contactor that can be operated frequently will cause the contact wear to intensify, resulting in damage to the electrical appliance; For plastic case circuit breakers, even if they are operated infrequently, it is difficult to meet the requirements. Therefore, in use, it is often used together with the starter in series in the main circuit. At this time, the contactor in the starter carries the assessment of connecting the starting current, while other electrical appliances only carry the assessment of breaking the motor overload current that usually occurs in operation. As for the protection function, it is completed by the supporting protection device

in addition, the control of the motor can also adopt the contactless mode, that is, the soft start control system. The main circuit of the motor is connected and disconnected by thyristor. In order to avoid the continuous loss of these components, some vacuum contactors are used to carry the load of the main circuit (parallel on the thyristor) in normal operation. This kind of control can be programmed or non programmed; Near control or remote control; Slow start or fast start and other methods. In addition, relying on electronic circuits, it is easy to achieve various protection functions such as electronic relays

motor protection device

the damage of the motor is mainly caused by the overheating of the winding or the reduction of insulation performance, and the overheating of the winding is often caused by the excessive current flowing through the winding. There are two main types of motor protection: current and temperature detection. The following is a brief introduction of the products

1. Current detection type protection device

(1 when the oil level height is less than 2/3 of the barrel depth, it should be replaced immediately) the thermal relay uses the load current to flow through the calibrated resistance element, so that the bimetal thermal element will bend after heating, so that the contact of the relay will act before the motor winding is burnt out. Its action characteristics are close to the allowable Overload Characteristics of the motor winding. Although the accuracy of the action time of the thermal relay is general, it can achieve effective overload protection for the motor. With the continuous improvement and improvement of the structural design, in addition to temperature compensation, it also has the functions of phase failure protection and load imbalance protection. For example, T series bimetal thermal overload relay introduced from ABB; 3ua5 and 3ua6 series bimetal thermal overload relays imported from Siemens; Jr20 and JR36 thermal overload relays, of which jn36 is a secondary development product, which can replace the obsolete product jrl6

(2) for motor protection with thermal magnetic trip, the thermal circuit breaker is used for overload protection. Its structure and action principle are the same as that of the thermal relay. Some of its bimetallic thermal elements directly push the tripping device after bending, and some make the contact connect, and finally cause the circuit breaker to disconnect. The setting value of electromagnet is high, and it only acts in case of short circuit. Its simple structure, small size, low price, action characteristics in line with the current standards, reliable protection, so it is still used by a large number of materials. The greater the driving force, especially the small capacity circuit breaker. For example, M611 motor protection circuit breaker imported from ABB, domestic dwl5 low-voltage universal circuit breaker (200-630a), s series molded case circuit breaker (100, 200, 400 in)

(3) the electronic overcurrent relay detects the fault current signal through the internal current transformers of each phase, and executes the corresponding action after being processed by the electronic circuit. The electronic circuit has flexible changes and diverse action functions, which can widely meet the protection of various types of motors. Its features are:

① multiple protection functions. There are three main types: overload protection, overload protection + phase failure protection, overload protection + phase failure protection

② the action time can be selected (in line with gbl4048.4-93 standard)

standard type (level 10):7.2in (in is the rated current of the motor), 4-1os action, for standard motor overload protection, quick acting type (level 10a):7.2in, 2-1os action, for submersible motor or compression motor overload protection. Slow acting type (level 30): 9-30s action at 7.2in, which is used for motor overload protection with long starting time, such as blower motor

③ wide current setting range. The ratio of the maximum value to the minimum value can generally reach 3-4 times, or even greater times (1.56 times for thermal relays), which is especially suitable for occasions where the motor capacity often changes (such as mines)

④ there is fault display. The LED displays the fault category, which is convenient for maintenance

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